1. Field of the Invention
The present invention relates to an air conditioning unit for a vehicle.
2. Description of the Related Art
Generally, a compressor of an air conditioning unit for a vehicle is operated intermittently to prevent excessive cooling and a subsequent failure in cooling due to the freezing of an evaporator. The intermittent operation of the compressor causes, however, a variation in the cooling capacity and varies widely the temperature of cooled air to be supplied into a passenger room of a vehicle. As a result, the cooling effect will be deteriorated.
The compressor of an air conditioning unit is driven by a vehicle engine, and since the intermittent operation is realized by the engagement and disengagement of a clutch unit disposed between the engine and the compressor, the smoothness of the driving of the vehicle is also deteriorated. Therefore, there is a demand for the provision of a low cost mechanism in which a compressor capacity (compressor displacement) is adjusted according to the cooling load, to minimize or eliminate intermittent operation of the compressor and to stabilize the temperature of cooled air to be supplied, thus improving the cooling effect in the passenger room as well as the smoothness of the vehicle driving.
In order to meet the above requirement, there is proposed a prior art example in which a spool valve type evaporating pressure regulator is disposed in a suction piping which forms a return circuit for the refrigerant of the vehicle air conditioning unit. The regulator adjusts the pressure of an evaporator according to the magnitude of the cooling load to maintain the temperature of evaporator above a predetermined value. However, the spool valve type evaporating pressure regulator has a drawback in that lubricant is not sufficiently supplied to the compressor of the air conditioning unit when the spool valve is in an almost completely closed state.